1. Field of the Invention
The present invention relates to a continuous method of purifying carbon nanotubes, and more particularly, to a continuous method of purifying carbon nanotubes under a sub-critical water or supercritical water condition.
2. Description of Related Art
A structure of the carbon nanotube (hereinafter, referred to CNT) was discovered firstly in 1991; and manufactures, physical properties and applications thereof have been accomplished actively. Further, it is confirmed that the CNT is prepared if transition metals such as Fe, Ni, and Co are added upon electric-discharging. The full-scale research is not initiated until considerable samples are produced by means of a laser vaporization method in 1996. Such CNT is in a shape of hollow tube having a graphite surface rolled up in a nano-sized diameter. At this time, the electric property of the CNT is classified into conductor or semi-conductor in accordance with degree and structure with which the graphite surface is rolled up. Further, the CNT can be classified into a single-walled carbon nanotube SWCNT, a double-walled carbon nanotube DWCNT, thin multi-walled carbon nanotube, a multi-walled carbon nanotube MWCNT, and roped carbon nanotube in accordance with the number of the graphite walls.
In particular, the CNT is superior in mechanical intensity and elasticity, and has chemical stability, environmental friendliness, as well as electrical conductivity and semi-conductivity. Further, the CNT has a diameter of 1 nm to several tens nm and a length of several μm to several tens μm so that it is greater than any exiting materials of which aspect ratio amounts to about 1,000. Further, since its specific-surface area is very large, it is under the spotlight in various fields such as future-generation information electronic material, high-efficiency energy material, high-capacity complex material, and environmental friendliness material.
However, it is difficult to utilize electric, mechanical and physical properties of the CNT due to impurities such as carbon substance, amorphous graphite, and alpha carbon besides the CNT which are prepared during the manufacturing process of the CNT. Therefore, it is necessary for a continuous method of purifying a large amount of CNT in order to enlarge the practical range in various uses while supporting the properties of the CNT. The technique of purifying the CNT is exemplified as a method of purifying the CNT using an acidic gas pyrolysis in Korean patent No. 2001-0049298, a method of purifying through the pyrolysis using an oxidizer in U.S. Pat. No. 5,641,466 and a method of purifying at a higher temperature using the oxidizer in Japanese Patent No. 1996-12310.
However, in cases of the above techniques, the time needed for heat treatment is long and steps up to an acid-treatment process are complicated, which results in consuming too much time.